1. Field of the Invention
This invention relates to an incandescent lamp comprising a glass envelope, a filament supported by constricted portions of the glass envelope, lead rods provided at both ends of the filament and sealing portions formed respectively by constricting portions of the glass tube, which surround the lead rods.
2. Description of the Related Art
In one example of double-end-sealed tube type incandescent lamps which have been used in the light sources for exposure of copying machines by way of example, an incandescent lamp is constructed in the following manner with reference to FIG. 4 of the drawings. A filament assembly composed of a filament 20 comprising light-emitting parts 21 and light-unemitting parts 22, which are arranged alternately, inner leads 71 connected to both ends of the filament 20 respectively, hermetically-sealing metal foils 72 connected to the inner leads 71 respectively, and outer leads 73 connected to the metal foils 72 respectively is disposed in a glass tube 70 made of quartz glass as illustrated in FIG. 4. Sealing portions 74 are formed by pinching portions of the glass tube 70, which surround the metal foils 72 respectively, to the metal foils 72 under heating and pressing. Contacts 40 are connected respectively to the outer leads 73 projecting out of the sealing portions 74. Cylindrical bases 50 are respectively arranged so as to surround the contacts 40 and exposed portions of the outer leads 73 and are fixed thereon with a cement 60 filled in each base 50.
Such conventional incandescent lamps are however accompanied by the following problems.
(1) Since each sealing portion is formed by making use of the metal foil 72, the sealing portion becomes longer. It is therefore impossible to make the effective emission length of the filament 20 sufficiently long to extend for the whole length of the incandescent lamp. It is hence difficult to shorten the whole length of the incandescent lamp.
In addition, since it is impossible to make the effective emission length sufficiently long as described above, a dimmer is required to obtain a desired luminous intensity distribution. The cost of equipment hence becomes higher.
(2) The contact type incandescent lamp such as described above is supplied with electricity through feeding means coming into contact with the external surfaces of both contacts 40 and inwardly pressing them while being held therebetween under pressure. However, since the sealing portions 74 are long and are formed thinly by pinching under heating and pressing, the strength of the sealing portions 74 becomes insufficient, whereby troubles such as breaks at the sealing portions arise.